What are the loadouts on each and what all stuff do you have already?
To be honest, the 475 is a perfectly serviceable machine. It's the same speed as a Quadra 700 and it can take less VRAM than a 700/900/950, but it'll still output to an 1152x870 display and you can clock them up to 33MHz, and you can add "full" 040s to get the FPU, which isn't beneficial for much, but it might be beneficial for a software dev workflow.
You'll also need to account for an LC-PDS Ethernet card in a 475, which the 700/900/950 have onboard, but those machines also need an AAUI transciever.
Other than that, the advantages of a 900 are that it's got a lot of slots, a higher memory ceiling, and a 5.25 bay, which might have a CD-ROM drive in it. A 475 will be able to use an external scsi cdrom drive, however.
It's pretty hard to go wrong in this era, but what specifically you want can guide what you might look for. Other good machines are the 630-640 series, which has a CD-ROM drive but can only do 832x624 video.
The 650/800 are sort of the successor to the 700, they both support CD-ROM but not all of them have it, they've mostly got onboard Ethernet (there are some 650s without, though) and they have varying CPU configurations, mostly with full 040s.
The 840 is also good, slightly faster stock CPU than the others, but it lacks the 950's RAM ceiling and the 800's memory interleaving, so if you want a top end machine, picking the one whose particular go-fast stripes match what you want to do.
Overall, of that group, my recommendation probably falls to the 475, provided you either don't need or can budget for adding things you want to it. (ethernet, CD-ROM in particular)
If you don't clock it or accelerate it you'll wait longer for, like, builds, or other "compute job" types of things, but day-to-day I don't know that i think you'll notice the difference between 25 and 33MHz, especially in system 7 and especially if you aren't multi-tasking or running background work like a chat or IM client. (For example, I had an 840av in 2002 or so I was using as my daily internet system, with 64-72ish megs of RAM. I probably benefited over a 25-33MHz system because I was running AIM and Outlook Express in the background most of the time.)